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David Cameron has indicated that he is ready to consider using money from the UK's official aid budget to fund military operations to stabilise war-torn states in the poor world.

The suggestion will alarm aid charities and NGOs (non-governmental organisations), who fear that the UK's achievement of the United Nations target to spend 0.7% of national income on overseas assistance will be undermined if any of that cash is diverted to defence activities.

As Whitehall departments fight it out for scarce resources in the upcoming Spending Review for 2015/16, Mr Cameron said it was right to look for ways in which the Department for International Development (DfID) can work more closely with the Foreign Office and Ministry of Defence.

Initiatives which provide the basic level of security needed for development to take place can be an "important" use of aid funds, he said.

With defence among a number of departments threatened with further cuts in the Spending Review, the Prime Minister is coming under pressure from Conservative backbenchers to drop the protection he has so far granted to the aid budget.

Overseas aid is one of a handful of priorities - alongside the NHS and schools - which have been shielded from cuts under Chancellor George Osborne's austerity programme, and are currently slated to be preserved in the Spending Review, due by the end of June.

Any cut in the DfID budget would threaten to breach Mr Cameron's pledge to meet the United Nations target of spending 0.7% of national income on overseas aid, which Britain will meet for the first time in 2013. But NGOs fear that the PM may meet his pledge while reducing the funds available for aid activities, by reallocating some of the responsibilities of the MoD or Foreign Office to DfID. Asked whether he felt there was room for money in the aid budget to be spent on defence activities, Mr Cameron said: "I think we have to demonstrate that the DfID budget is spent wisely."

International Development Secretary Justine Greening was "rightly keen" to focus on countries which have been affected by conflict and war, none of which have achieved any of the UN Millennium Development Goals for 2015, he said. "We should be thinking very carefully about how we help states that have been riven with conflict and war," said Mr Cameron.

Oxfam's head of policy Max Lawson said the aid budget should be spent on "hospitals and not helicopter gunships". He told BBC Radio 4's Today programme: "The millions of people up and down the country who support the fantastic stance the Government is taking, protecting the aid budget when every other G8 nation is not doing that - they expect this to be spent on schools and not soldiers. So we cannot see any penny diverted into the military."

But Tory MP Patrick Mercer, a former Army officer, said security and overseas aid were "inextricably linked". He told Today: "I absolutely understand the need for overseas aid, although not necessarily at this level. But I do see that so much of the aid we have attempted to spend in the past in places like Iraq and Afghanistan has not been delivered as effectively as it might because of the lack of a benign security environment. If that means more money has to be spent on defence in order to increase the efficacy of overseas aid spending, I'm all for it."